1. Technical Field of the Invention
The present invention relates to the simultaneous measurement of two or more gases using optical path switching. More specifically, it relates to such measurement using dual beam spectroscopy, including gas filter correlation radiometry.
2. Discussion of the Related Art
Optical path switching has many potential applications, particularly in the field of dual beam spectroscopy. In dual beam spectroscopy, light from a radiation source traverses a measurement path and is then divided between two optical paths. Each optical path generally contains some medium through which the radiation is transmitted and thus partially absorbed and/or reflected. The key measurement in this type of spectroscopy is related to the intensity difference of the radiation that takes these two paths. For illustrative purposes, a gas filter correlation radiometer (GFCR), one example of a dual beam spectrometer, will be discussed in detail.
Gas filter correlation radiometers (GFCRs) may infer the concentration of a gas species along some measurement path either external or internal to the GFCR. In many GFCRs, gas sensing is accomplished by viewing alternately through two optical cells the emission/absorption of the gas molecules along the measurement path. These two optical cells, often called the correlation and vacuum cells, are an example of the media found in the two optical paths of a dual beam spectrometer. The correlation cell contains a high optical depth of gas species i and thus strongly absorbs radiation at the molecular transition wavelengths of the particular gas. In effect, the correlation cell acts as a spectral "notch filter" to the incoming radiation, the spectral notches being coincident with the band structure of gas species i. The vacuum cell generally encloses a vacuum or a gas or gas mixture exhibiting negligible or no optical depth, e.g., nitrogen, an inert gas, or even clean dry air. The difference in signal between these two views of the emitting/absorbing gas species i within the spectral region of interest plus, or in combination with, the sum of the signals of these two views can be related to the concentration of this gas along the measurement path.
In one known GFCR for measuring a single gas concentration in a particular quantity disclosed in U.S. Pat. No. 5,128,797, issued to Sachse et al. and assigned to the National Aeronautics and Space Administration (NASA), the specification of which is hereby incorporated by reference, a non-mechanical optical path switch comprises a polarizer, polarization modulator and a polarization beam splitter. The polarizer polarizes light from a light source into a single, e.g., vertically polarized, component which is then rapidly modulated into alternate vertically and horizontally polarized components by a polarization modulator. The polarization modulator may be used in conjunction with an optical waveplate. The polarization modulated beam is then incident on a polarization beam splitter which transmits light of one orthogonal component, e.g., horizontally polarized, and reflects light of a perpendicular component, e.g., vertically polarized. In a gas filter correlation radiometer application, the transmitted horizontally polarized beam is reflected by a mirror, passes through a gas correlation cell, and is transmitted through a second beam splitter. The reflected vertically polarized beam passes through a vacuum cell, is reflected by a mirror and then reflected by the second beam splitter. The beam combiner recombines the horizontal and vertical components into a single beam which is read by a conventional detector. This approach has numerous advantages, such as no mechanical means being required to alternate the view of the detector through the correlation and vacuum cells, fast response, etc.
It would be desirable, in numerous applications, to be able to measure two or more gas concentrations simultaneously, either independently or non-independently, with a single device using an optical path switch. It further would be desirable to do such measurement with optimal optical balance.